To get up-to-date GRS longitudes, you should download the following
file to your Guide folder. (This can apply to Guide 8 and 9 users.)
You already have a copy of this file in your Guide folder, and it
will give a decent GRS longitude already. But the above will include
There are comments at the end of this file... I'll cut and paste them
below; it should give a good idea as to why I removed the option in Guide
to set the GRS longitude by hand.
At one point, the GRS longitude in Guide was set by the user.
But the GRS moves; what is really needed is a table giving the
GRS longitude as a function of time. This file provides that table.
Guide computes the System II GRS longitude for any date by
interpolating within this table. Data are given from most recent to
oldest. For each date, a year, month, day, and System II
longitude are given, followed by the source of the data. For dates
after the most recent data point, the last GRS longitude is used;
for dates preceding the table, the oldest GRS longitude is used.
Note that, while coverage is good for years since 1986, the data
gets a little sparser for previous dates.
The GRS longitude currently appears to be increasing at about
nine degrees per year (with plenty of fluctuations). I've added a
totally fictitious longitude for 2030 based on that drift, just to
get around the fact that otherwise, the GRS would remain at the last
listed value, which is almost certainly even less realistic than
the assumed drift.
Data since 2009 has come from JUPOS, the database for object
positions on Jupiter.
1986 to 2009 longitudes came from _Sky & Telescope_, which in turn
got the data from ALPO (Association of Lunar and Planetary Observers).
I checked a random sampling of my back issues of that magazine to
compile the table. Because S&T is published about two months before the
date given on the cover, I've assumed that the longitude given in (for
example) the August issue of a given year is the one that was actually
measured in June, and have arbitrarily chosen the first of the month.
Jari Suomela found some data in Bertrand Peek's book _The Planet
Jupiter_ for years 1831 to 1952. See 'grs_peek.cpp' for details.
Other sources would be welcome. Seems to me as if somebody, somewhere,
must have assembled a table going back to Galileo's observations...